This invention relates to an improved process for the production of release papers for the casting of organic thermoplastic film organic solvent solutions. A preferred use of the paper of the invention is for the casting of polyvinylbutyral film, which film may be used in preparing reflective products such as highway signs.
Release or stripping papers have long been used for the casting of sheets, films or transfers of thermoplastic resins. Conventionally, the papers have been coated with a release agent, such as a silicone, starch, silicate or the like which provides a release layer between the cast plastic surface and the paper. Thus, as in the conventional casting process, solvent solutions of thermoplastic resins could be coated on the release coated paper, dried to remove the solvent, and then dried film or sheet could be easily stripped from the release surface. The surface of the thermoplastic resin film is a replicate of the release surface. Endless sheets or films of the thermoplastic resin can be produced, if desired, by this technique. Typical examples of coated release papers are disclosed in the following patents.
U.S. Pat. No. 3,067,054--(1962) to Reese PA1 U.S. Pat. No. 3,331,729--(1967) to Danielson et al. PA1 U.S. Pat. No. 3,519,456--(1970) to Reed et al. PA1 U.S. Pat. No. 3,873,390--(1975) to Cornell et al. PA1 U.S. Pat. No. 3,896,249--(1975) to Keeling et al. PA1 U.S. Pat. No. 3,952,131--(1976) to Sideman.
U.S. Pat. No. 3,519,456 discloses various types of papers, including glassine, vegetable parchment and kraft, coated or impregnated with a cured siloxane or Werner type chromium complex, or polyethylene, tetrafluoroethylene or polypropylene plastic film as a carrier sheet for casting a transferable ink composition. U.S. Pat. No. 3,873,398 discloses a silicone coated release paper for use in, for example, the production of reflective thermoplastic sheets for use in highway signs. Stripping or release papers having a polyethylene coating and being further coated with a release agent are disclosed in U.S. Pat. Nos. 3,331,729, 3,896,249 and 3,952,131. U.S. Pat. No. 3,067,054 discloses the use of paper coated with low molecular weight (MW=12,000 to 20,000) polyethylene for solution casting of certain vinyl thermoplastic resin films.
Recently, attempts have been made to produce a polyethylene extrusion coated paper which could be used in casting thermoplastic films such as polybutyral films. Polyethylene coated papers are conventionally produced by continuously extruding a hot film of polyethylene from an extruder and adhering the film of polyethylene while still hot to a paper substrate by passing the hot film and paper substrate through a nip of a pair of press rolls. Extrusion melt temperatures of 575.degree. F. or above are conventionally used in this process. Cooling of the adhered film was accomplished by maintaining the roll contacting the hot film at a cool temperature. Polyethylene coated papers made by such conventional methods did not have good release properties for cast polyvinylbutyral films and in some cases attempts to strip the polyvinylbutyral film from the release paper would delaminate the polyethylene release layer from the paper. By (1) lowering of the temperature of the polyethylene melt, (2) compensating for the loss of adhesion between the paper and the polyethylene film due to lower extrusion temperatures by applying a layer of an adhesion primer to the paper substrate and (3) extrusion coating with a low density polyethylene, as by the process of this invention, a paper having excellent release properties can be produced. This release paper has a mirrorlike release surface free of defects and can be used in particular where a cast film requires a reflective surface as in the production of, for example, highway reflective signs.